[phobos] array.shift for deletion/insertion

[spir]

Hello,

There is no builtin func to delete or insert one or more elements from or into 
a dynamic array. The reason is probably that providing a dedicated idiom may 
let such an operation look cheap (same reason as for refusing 'in' for dyn arrays).
Still, there is a need for this functionality, to avoid
* either repetedly coding it when needed, possibly with bugs,
* or using concat, which copies the whole content instead of the necessary part.

For both operations, the actual process is a shift operation on the part of the 
array ranging from a lower bound to the upper end, for an offset equal to the 
deleted/inserted slice's length. We could, I guess, provide this shift process 
builtin. This would allow writing deletion/insertion easily, while making it 
obvious that the operation is costly.
See example below. What do you think?


Denis

void shift (T) (ref T[] array, size_t low, sizediff_t offset) {
     if (offset == 0) return;
     auto len = array.length;
     size_t i;

     // move up for insertion: proceed backwards
     if (offset > 0) {
         array.length = len + offset;
         for (i=len-1 ; i>=low ; i--)
             array[i+offset] = array[i];
     }
     // move down for deletion: note offset is negative!
     else {
         for (i=low ; i<array.length ; i++)
             array[i+offset] = array[i];
         array.length = len + offset;
     }
}

unittest {
     auto a = [0,1,2,3,4,5,6,7,8,9];
     writeln(a);
     // delete [4..6]
     a.shift(7, -3);
     writeln(a);	        // [0,1,2,3,7,8,9]
     // insert [4..6]
     a.shift(4, +3);
     a[4..7] = [4,5,6];
     writeln(a);         // [0,1,2,3,4,5,6,7,8,9]
}

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